Applications sometimes need to establish and manage a session between computing devices. A session is a set of interactions between computing devices that occurs over a period of time. As an example, real-time communications applications such as MICROSOFT MESSENGER or Voice over Internet Protocol (“VoIP”) establish sessions between communicating devices on behalf of users. These applications may use various mechanisms to establish sessions, such as a “Session Initiation Protocol” (“SIP”). SIP is an application-layer control protocol that devices can use to discover one another and to establish, modify, and terminate sessions between devices. SIP is an Internet proposed standard. Its specification, “RFC 3261,” is available at <http://www.ieff.org/rfc/rfc3261.txt>. A specification for extensions to SIP relating to event notifications, “RFC 3265,” is available at <http://www.ieff.org/rfc/rfc3265.txt>. A specification for locating SIP servers, “RFC 3263,” is available at <http://www.ietf.org/rfc/rfc3263.txt>. All three of these specifications are incorporated herein in their entirety by reference.
Applications may use SIP with another protocol to send or receive information. As an example, an application may use SIP with Real-time Transport Protocol (“RTP”) for transporting real-time data during a session. By using SIP with other protocols, applications can create and manage a session and exchange information during the session. The protocol used with SIP to exchange information may segment the information into messages. As an example, a VoIP application may segment a long narration into shorter messages. Exchanging messages during a session is referred to as a “dialog.” SIP may use lower-level communications layers to transport a dialog's messages, such as Transmission Control Protocol/Internet Protocol (“TCP/IP”), which are commonly employed transport- and network-layer protocols.
A SIP network comprises entities that can participate in a dialog as a client, server, or both. SIP supports four types of entities: user agent, proxy server, redirect server, and registrar. User agents initiate and terminate sessions by exchanging messages with other SIP entities. A user agent can be a user agent client, which is generally a device that initiates SIP requests (e.g., to initiate a session), or a user agent server, which is a device that generally receives SIP requests and responds to such requests. As examples, “IP-telephones,” personal digital assistants, and any other type of computing device may be user agents. A device can be a user agent client in one dialog and a user agent server in another, or may change roles during the dialog. A proxy server is an entity that acts as a server to clients and a client to servers. In so doing, proxy servers intercept, interpret, or forward messages between clients and servers. Proxy servers contribute to network security by, e.g., validating senders and recipients of messages. A redirect server accepts a SIP request and generates a SIP response directing the client that sent the request to contact an alternate network resource. As an example, a redirect server may indicate at which of several devices a particular user is presently available. A registrar is a server that accepts registration information from SIP clients and informs a location service or other entities of the received registration information.
SIP supports two message types: requests, which are sent from a client to a server, and responses, which are sent from a server to a client, generally when responding to a request. A SIP message comprises three parts. The first part of a SIP message is a “start line,” which includes fields indicating a message type and a protocol version. The second part of a SIP message comprises header fields whose values are represented as name-value pairs. The third part of a SIP message is the message's body, which is used to describe the session to be initiated or contain data relating to the session. Message bodies may appear in requests or responses.
SIP messages are routed based on the contents of their header fields. To be valid, a SIP request should contain at least the following six header fields: To, From, CSeq, Call-ID, Max-Forwards, and Via. The To header field indicates the logical identity of the recipient of the request. The From header field indicates the logical identity of the initiator of the request. The Max-Forwards header field indicates the number of hops a request can make before arriving at its destination. As an example, if a message from device A transits device B before arriving at destination device C, the message is said to have made two hops (e.g., devices B and C). The Via header field indicates the path taken by the request so far (e.g., a sequence of network addresses of devices through which the request has transited) and indicates the path that should be followed when routing the response. A header may also contain Record-Route fields that are used to indicate that future requests and responses should be routed through an indicated device. Network devices may insert Record-Route header fields specifying devices when forwarding a SIP message in an attempt to force subsequent messages in a dialog to be routed through the specified devices. The Record-Route header field may contain an identifier (e.g., network address) for the device and parameters. These and other header fields are described in the SIP specifications referenced above.
Several organizations may each offer SIP servers as a network resource. These organizations may enable SIP clients and servers from outside their networks, e.g., from the Internet or other organizations, to connect to their SIP servers to exchange messages. An organization may acquire a certificate for its SIP server from a trusted entity to enable other organizations to authenticate messages its SIP server sends. The SIP server may add an indication of this certificate to messages it sends using a protocol such as Transport Layer Security (“TLS”).
Organizations may wish to federate their SIP servers or networks. By federating their SIP servers or networks, organizations enable participants in their networks to communicate with SIP servers of other trusted organizations in the federation. As an example, an organization may desire to set up an application-level virtual private network (“VPN”) to enable applications being executed by the organization to securely communicate with SIP servers that may be located in another organization's network. An effective approach to enable administrators of SIP servers to securely federate their SIP servers would have significant utility.